MySQL で既存のフィールドを一意にしますか?

mysql> create table UniqueConstDemo
- > (
- > name varchar(100) unique
- > );
Query OK, 0 rows affected (0.72 sec)

mysql> insert into UniqueConstDemo values('John');
Query OK, 1 row affected (0.19 sec)

mysql> insert into UniqueConstDemo values('John');

mysql> insert into UniqueConstDemo values('John');
ERROR 1062 (23000): Duplicate entry 'John' for key 'name'

mysql> insert into UniqueConstDemo values('Bob');
Query OK, 1 row affected (0.11 sec)

mysql> select *from UniqueConstDemo;